As discussed in this bug report the current behavior (12.04) of the messaging menu plugin for thunderbird is to turn the envelope blue only for certain types of emails (e.g. depending on priority, mailing lists, etc). Typically mailing lists should not change the icon color.

You can filter your emails in thunderbird (for instance, change priority) so as not to trigger the notification. To do this, setup a new filter from Tools> Message Filters. Provide criteria that match messages you deem of low priority. Among the possible actions to perform, there is Set priority to. Pick it up, then set value to Low or Lowest.

You can check if it works by adding "you as sender" to the criteria and sending yourself a message.

At the moment the user has no way to indicate explicitly which messages should turn the icon blue. Please consider the above as a workaround, as I believe priority should be set only on the sender side.